Field of the Invention
The present invention relates to an image pickup lens to be mounted on imaging devices adopting relatively small and thin solid-state imaging elements such as CCD sensors or C-MOS sensors mounted on portable terminals such as cellular phones and smartphones or PDAs (Personal Digital Assistants).
Description of the Related Art
Recently, most portable terminals such as cellular phones and smartphones, PDAs and other devices are readily equipped with a camera function. With the aim to enhance the portability and usability of these devices, further downsizing and thinning of the devices have been considered, and at the same time, the improvement of the camera function corresponding to increased number of pixels has also been considered. In response to this trend, further downsizing and increase in the number of pixels are realized in imaging elements adopted in the imaging devices mounted to such devices. Furthermore, the image pickup lenses disposed in these imaging devices are required to not only have high resolving power to correspond to the increased number of pixels but also realize downsizing and thinning. Moreover, there are strong demands for an image pickup lens having a bright lens system and a wide angle of field capable of taking an image of an object from a wide area, and corresponding to the highly dense imaging elements.
In the prior art, a large number of image pickup lenses having a three-lens configuration capable of correcting aberrations to a certain level and preferable from viewpoints of size and costs had been adopted as image pickup lenses mounted on the above-listed devices, but along with the increase in the number of pixels of the imaging elements, image pickup lenses adopting a four-lens configuration realizing a higher performance than the three-lens configuration are becoming popular. Recently, however, along with the further increase in the number of pixels of imaging devices, devices having a camera function with pixels exceeding far beyond 5 megapixels are provided. In response to such trend of increase in the number of pixels, image pickup lenses having a five-lens configuration capable of realizing even higher resolution and higher performance than the four-lens configuration have been proposed.
For example, Japanese Patent Laid-Open No. 2007-264180 (Patent Document 1) discloses an image pickup lens having, in order from an object side, a positive first lens having a convex object-side surface, a negative second lens having a meniscus shape with a concave surface facing an image side, a positive third lens having a meniscus shape with a convex surface facing the image side, a negative fourth lens having both surfaces formed as aspherical surfaces and having the image-side surface concaved near an optical axis, and a positive or a negative fifth lens having both surfaces formed as aspherical surfaces.
Further, Japanese Patent Laid-Open No. 2011-085733 (Patent Document 2) discloses an image pickup lens system having, in order from an object side, a first lens group including a first lens having a convex shape on the object side, a second lens group including a second lens having a concave shape on an imaging side, a third lens group including a third lens having a meniscus shape and a concave surface arranged on the object side, a fourth lens group including a fourth lens having a meniscus shape and a concave surface arranged on the object side, and a fifth lens group including a fifth lens having a meniscus shape that has an aspherical surface with an inflection point arranged on the object side.
According to the image pickup lens disclosed in Patent Document 1, a high-performance image pickup lens system having a five-lens configuration capable of effectively correcting axial chromatic aberration and chromatic aberration of magnification and responding to the increase in the number of pixels is realized by optimizing the materials of the lenses and the surface shapes of the lenses. However, the total track length of the lens system is approximately 8 mm, which still leaves a problem in applying the lens to devices requiring further thinning. Further, since the F-value of the lens is approximately 2.8 and the angle of field is approximately 32°, it cannot be said that the disclosed lens provides a sufficiently bright lens system or a wide angle of field that is required in recent lens systems.
The image pickup lens disclosed in Patent Document 2 realizes a high resolving power and a total track length of approximately 6 mm, which realizes relative downsizing and thinning in a lens system. However, the F-value of the lens is approximately 2.8 and the angle of field is approximately 32°, which means that the image pickup lens described in Patent Document 2 cannot sufficiently satisfy the specifications (high resolution, downsizing, thinning, bright lens system, and wide angle of field) required in recent lens systems.